In many diagnosis and interventional procedures, it is often necessary to determine the location of a medical probe or catheter relative to a location of interest within the body. In interventional cardiac electrophysiology procedures, for example, it is often necessary for the physician to determine in real-time the location of a medical probe such as a electrophysiology mapping catheter or therapeutic delivering catheter (e.g., an ablation catheter) relative to the patient's internal anatomy. During such procedures, the physician may deliver the mapping catheter through a main vein or artery into an interior region of the heart to be treated. Using the mapping catheter, the physician may then determine the source of the cardiac rhythm disturbance or abnormality by placing a number of mapping elements carried by the catheter into contact with the adjacent cardiac tissue and then operating the catheter to generate an electrophysiology map of the interior region of the heart. Once a map of the heart is generated, the physician may then advance an ablation catheter into the heart, and position an ablating element carried by the catheter tip near the targeted cardiac tissue to ablate the tissue and form a lesion, thereby treating the cardiac rhythm disturbance or abnormality.
The navigation of medical probes such as mapping and ablation catheters has traditionally been accomplished using fluoroscopic techniques in which radiopaque elements located at or near the distal end of the probe are used to fluoroscopically image the probe as it is routed through the body. Such systems produce a two-dimensional image of the probe, as represented by the illuminated radiopaque element, allowing the physician to ascertain the general location of the probe. Although fluoroscopy is commonly used in EP procedures, such technique does not permit the imaging of soft tissues, making it difficult for the physician to visualize features of the anatomy as a reference for navigation.
Various ultrasound-based imaging catheters and probes have been developed for directly visualizing medical probes in applications such as interventional cardiology, interventional radiology, and electrophysiology. For interventional cardiac electrophysiology procedures, for example, ultrasound imaging devices have been developed that permit the visualization of anatomical structures of the heart directly and in real-time. In some electrophysiology procedures, for example, ultrasound catheters may be used to image the intra-atrial septum, to guide transseptal crossing of the atrial septum, to locate and image the pulmonary veins, and to monitor the atrial chambers of the heart for signs of a perforation and pericardial effusion. Many ultrasound-based imaging devices are designed to image in the far-field at a distance greater than about 1 cm, allowing the physician to visualize anatomical structures, the position of devices relative to those structures, as well as any anomalies or interesting characteristics of those structures. These devices typically operate at lower ultrasonic frequencies of between about 2 to 15 MHZ in order to balance far-field tissue/blood penetration against far-field resolution and image quality.
In some procedures, it may be desirable to visualize tissue that is in close proximity to the imaging device (e.g., at or less than about 1 cm) in order to determine the characteristics of that tissue. For example, such feedback may help the physician to determine whether the device is in contact with tissue, to determine whether the tissue is healthy tissue or scar tissue, to determine the thickness of the tissue, to determine whether an ablation lesion is transmural or continuous with adjacent lesions, as well as other characteristics.